The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.
The present invention relates to an improved process for treating or converting certain metal-loaded ion-exchange resin microspheres into microspheres consisting of a porous carbon or graphite matrix containing a dispersed metal carbide or metal oxide-carbide phase. In the context of this invention a microsphere refers to a spheroidal particle ranging from 5 to 2000 microns in diameter; ion-exchange resin refers to spheroidal-shaped resins, which may consist in one case of porous cation-exchange resins having exchange sites, said resins consisting of copolymers of acrylic or methacrylic acid and divinylbenzene. Such resins are available commercially as represented by Amberlite IRC-72, available from Rohm & Haas Company, Philadelphia, Pa.
It has previously been proposed to utilize carbonized or graphitized fissile and/or fertile metal-loaded ion-exchange resin microspheres as nuclear fuel where the metal is selected from uranium, thorium, or plutonium, or mixtures thereof and where the metal exists as an oxide, carbide, or oxide-carbide mixture dispersed within a carbon or graphite matrix. A process for the formation of microspheroidal nuclear fuels derived from spheroidal ion-exchange resin beads or microspheres is described in U.S. Pat. 3,880,769 of common assignee. Resin-derived fuels have the potential merit of being fairly cheap to produce, of resulting in a porous spheroidal product, and of being amenable for deposition of pyrolytic carbon coatings to retain fission products. The coated particles are amenable for reprocessing for recovery of unburnt fuel and separation from fission products. There are, however, operational difficulties in forming carbide fuels derived from ion-exchange resins which must be overcome in order to realize the full potential of such fuels.
In order to make a carbide fuel from a metal-loaded resin it must be heated (i.e., converted) at a temperature in the range 1200.degree.-2000.degree. C. in an inert atmosphere such as argon or helium. Carbonization up to 1200.degree. C produces a metal oxide dispersion in a carbon matrix; further heat treatment up to 2000.degree. C. converts the oxide to carbide where the rate of conversion in a fluidized bed depends on such factors as temperature, which determines the partial pressure of CO, and specific sweep rate or gas flow rate relative to batch size. Experience has shown that conversion of the metal-loaded resin at carbide-forming temperatures can result in extensive particle agglomeration accompanied by loss of sphericity and porosity. These effects are attributable to sintering of the oxide or carbide components of the carbonized resin. Moreover, since different increments of a given charge of imperfectly fluidized resin particles are subject to slightly varying degrees of sintering, the resulting particles can be non-uniform in size, shape, porosity, and composition. It is important in this technology that a given charge of product microspheres be as uniform as possible in these respects.
It is accordingly a general object of this invention to provide a process by which the aforementioned adverse sintering effects are avoided.
A principal object of this invention is to provide a process for converting spheroidal or spherical metal-loaded resin microspheres to spheroidal or spherical particles consisting of a porous carbon or graphite matrix containing a dispersed phase of a metal carbide MC.sub.x, where x is a number ranging from 1 to 2, or a mixture of MC.sub.x with MO.sub.2, where M may be any metal selected from the group having an automatic number in the range 58-71, 90-105, boron, cadmium, or any heavy metal carbide which may be usefully incorporated within a porous carbon matrix as a dispersed phase.
Another object is to provide uniform particles of the character described.
Nuclear fuel particles designed for use in high-temperature gas-cooled reactors (HTGR) may consist of kernels of a fissile material, uranium, or a fertile material, thorium, existing as an oxide or carbide or a mixture of oxide and carbide where the kernel is first coated with low-density so-called buffer carbon layer deposited from a decomposed hydrocarbon gas such as acetylene diluted with an inert gas such as argon or helium in a fluidized bed coater. By changing the coating conditions, a second high-density isotropic carbon coating can be deposited to form a product BISO-coated particle. If a TRISO-coated particle is desired, the additional SiC and carbon layers can be deposited in the same or separte operations. In one form of coated particle, a dense UO.sub.22 kernel or microsphere is used which is formed by known sol-gel processes. The sol-gel process involves a series of complex fabrication sequences which require close control of process parameters and involves fairly high fabrication costs. On the other hand, fissile or fertile microspheres made from spheroidal ion-exchange microspheres offer the promise of low fabrication cost since the original ion-exchange miscrosphere is preformed and available commercially as a low-cost off-the-shelf product. Because the metal-loaded microspheres are porous, they can provide an internal volume to accommodate fuel swelling and fission product gas which develops during reactor operation. It is therefore an additional object of the present invention to provide an improved fissile or fertile coated particle having a porous fissile or fertile carbide-containing kernel and to provide a method for obtaining the same. A further object is to teach a process for converting a metal-oxide-loaded porous resin microsphere to a carbide-loaded microsphere in which a substantial percentage of the porosity and sphericity of the resin microsphere is maintained during the conversion.